What is your favorite frightening music? For me it is probably Giyorgy Ligeti's Requiem, which has always reminded me of the throes of death, or someone alternately rising above life into transcendent spaces and then falling back into strangling horror:

Hardly an original choice, but given that it's also arguably my first exposure to the horror genre by way of Disney's Fantasia:

Mussorgsky's Night on Bald Mountain

It's not as emotionally consistent as, say, the Ligeti Requiem above, but that's kind of what I dig about it. There's majestic terror and there's creeping horror. There are manic sections of barely contained chaos, and then there're stately and subtly haunting sections. Broadly, it strikes many of the notes across the emotional spectrum that draw me to the horror genre in the first place.